System and method for rotational motion capture of a solid

ABSTRACT

A capture device of the orientation of a solid comprising a sensor of angular position, capable of being affixed to the solid and supplying a measuring datum related to a real orientation of the solid, wherein the sensor is sensitive to gravity/magnetic field; calculation means for generating test data independent of any measuring datum and related to an estimated orientation of the solid; and means for modifying the estimated orientation by confrontation of the measuring datum and test data. A process including capturing a measuring datum related to a real orientation of the solid, the measuring datum originating from an angular position sensor sensitive to gravity/a magnetic field; establishing test data independent of any measuring datum and related to an estimated orientation of the solid; confronting the test data and the measuring datum; and establishing a new estimated orientation of the solid based on the result of the confrontation.

STATEMENT OF RELATED APPLICATIONS

The present application is a continuation of pending U.S. patent application Ser. No. 10/509,987 filed Nov. 30, 2004 in the name of inventors Dominique David and Yanis Caritu, which, in turn, claims the benefit of priority based on International Patent Application No. PCT/FR03/01025, filed on Apr. 2, 2003, which is based off of French Application No. 02/04260 filed Apr. 5, 2002 in the name of inventors Dominique David and Yanis Caritu, entitled “Device for Rotational Motion Capture of a Solid”, all commonly owned herewith.

TECHNICAL FIELD

The subject matter described herein is directed to a device and a process for sensing the orientation and the rotational motion of a solid.

BACKGROUND

Motion sensors, and more precisely angular position sensors, are greatly miniaturized and are the focus of research to impart them with a robustness and cost compatible with applications for the public at large.

The position of a solid in space is entirely determined by the knowledge of six magnitudes. Examples of these are three magnitudes capable of translating translations and three other magnitudes capable of translating rotations. The three latter magnitudes correspond to angular positions. These can be utilized to determine motions known as skew motion, pitch motion and roll motion.

According to the envisaged applications it is not always necessary to place all six associated magnitudes at six degrees of liberty. A more restrained number of data can in effect suffice in a large number of cases.

Two types of sensors are mainly known which are capable of detecting the angular position or the rotation of a solid. These are on the one hand sensors sensitive to a magnetic field, such as magnetometers, and on the other hand sensors sensitive to acceleration, such as accelerometers. In an advantageous manner the accelerometers can measure any accelerations of the solid, thus for example, modifications of the orientation of the solid relative to the direction of the field of terrestrial gravity.

The magnetometers can be utilized in combination with an artificial source of magnetic field. All the same it is preferred to make use of magnetometers capable of detecting the orientation of the solid relative to the terrestrial magnetic field. It is considered of course that the directions of the magnetic field and of the acceleration of the apparent gravity are not collinear.

The sensors can be of the type having a single axis, that is, sensitive according to a single direction unique of space. However, sensors having two or three non-parallel axes are preferred. These supply measuring values allowing an angular position of a solid to which they are solid to be recognized completely. The sensors supply a measuring signal M which is connected to their inclination I by a function f such as: M=f(I)

The inclination is considered relative here to an angular reference position. The latter can be arbitrary or adjusted on the magnetic field or the terrestrial gravitational field. The magnitude to be known is the inclination I, which can be retrieved by calculation according to a formula I=f¹(M).

The inverse function f¹ is however difficult to establish with exactitude. In addition, it suffers from discontinuities and non-linearities. A difficulty is attached for example to the fact that the sensors using the field of apparent gravity do allow rotations to be found at any instant about horizontal axes but not about the direction of apparent gravity. The same applies to magnetometers which are efficacious only for measuring rotations whereof the axis is not confused with the direction of the magnetic field used as reference. Non-linearities also originate from trigonometric functions brought up by calculating the inverse function.

Additional inaccuracies originate from the fact that the sensors having three axes do not always have a very precise angular relation between the axes. For example, the axes are not exactly orthogonal.

OVERVIEW

The subject matter described herein is directed to a device and a capture process of the orientation of a solid not having the abovementioned limitations and difficulties. An aim in particular is to propose such a device which is low in cost and capable of being integrated into equipment destined for a wide public. Another aim is to propose a reliable device, little sensitive to phenomena of non-linearity affecting measurements, and allowing possible imperfections in the sensors to be taken into account directly.

To attain these aims more precisely concerns a capture device of the orientation of a solid which comprises a capture device of the orientation of a solid which comprises a sensor of angular position, capable of being affixed to the solid and of supplying at least a measuring datum related to a real orientation of the solid, wherein the sensor of angular position being sensitive to at least one of gravity or a magnetic field; calculation means for generating test data independent of any measuring datum and related to an estimated orientation of the solid; and means for modifying the estimated orientation of the solid by confrontation of the measuring datum and the test data.

According to a particular realization of the device, the modification means of the estimated orientation can comprise a first comparator connected on one side to the sensor and on the other side to the test data generator means. The first comparator thus receives the measuring datum and a test datum, and can establish at least one difference between the test datum and the measuring datum. The difference between the test datum and the measuring datum constitutes measuring the pertinence of the estimated orientation.

An aspect relates to a process for estimating the orientation of a solid comprising the following stages: input of at least one measuring datum originating from at least one angular position sensor and the establishment of at least one test datum representative of an estimated orientation of the sensor, b) confrontation of the test datum and of the measured datum, establishment of at least a new test datum representative of a new estimated orientation of the solid, corrected as a function of the preceding confrontation, and repetition of confrontation and establishment. Confrontation and establishment can be repeated until the confrontation reveals a difference between the test datum and the measuring datum less than a determined threshold. The confrontation of the data can comprise their comparison or the calculation of a difference, as shown hereinabove.

The described differs from the devices of the prior art by the fact that the determination of the orientation is not necessarily done in constant time. In the devices of the prior art, the determination of the orientation is effected in a fixed time corresponding to the necessary calculation time. In the case of iterative confrontation such as indicated hereinabove, the time taken by the determination of the orientation is, for example, associated with pertinence of the initial estimation of the orientation and the speed of convergence of successive estimations. In other terms, the time given for determining the orientation depends on the number of repetitions of confrontation and establishment. The processing time does not however constitute an obstacle for implementing the process. In fact the real measurements made are of the order of 500 per sensor and per second. It is thus possible to effect several estimation loops for each measurement. The number of loops is generally less than 30. Often, a few loops suffice. As indicated hereinabove, during establishment, a correlation calculation can be made according to a method of error gradient descent. Even though this constitutes a less preferable solution it is still possible to make random estimations.

Other characteristics and advantages of the invention will emerge from the following description, in reference to attached diagram. This description is given purely by way of illustration and not limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 illustrates a block diagram of the system in accordance with an embodiment.

DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 illustrates a block diagram of the system in accordance with an embodiment. The references 10 a and 10 b in FIG. 1 indicate respectively an accelerometer and a magnetometer. These are sensors with three axes of sensitivity, of known type, capable of providing measuring data representative of the orientation, that is, an angular position, of a solid S. The solid S is indicated summarily by broken lines. This is for example a part of the human body, the motions of which are to be studied, a computer mouse, a surgical instrument.

The measurements of the sensors, designated Θ_(m) are of scalar or vectorial magnitudes. They are representative, for example, of angles of skew, roll and pitch (φ,ψ,θ). These measurements are directed towards a comparator 12. This is, in the example illustrated, a differentiator. The comparator 12 also receives one or more test data Θ_(t) provided by a calculator 14. The test datum can be vectorial in type and can express angles according to several axes. The calculator 14 is utilized as means for generating test data. The test data are representative of an estimated orientation of the solid which can be random or not. These are, for example, triplets of angles of skew, roll and pitch (φ,ψ,θ). The calculator can be localized on the solid S.

The comparator provides a difference ΔΘ which, according to one or more axes, represents a spread between the real orientation, corresponding to the measuring datum, and the estimated orientation corresponding to the test datum. This spread can be used to refine the estimated orientation of the sensor, and thus of the solid to which it is fixed.

Nevertheless, it is possible to fix a threshold beyond which it is considered that the estimated orientation is sufficiently close to the orientation measured to be validated. This can take place by means of a second comparator 16 provided to compare the difference ΔΘ with the threshold value. When the difference is less than the threshold in absolute value the test datum Θ_(t) that is, the estimation of the angular position, is directed towards an output O.

However, when the difference is greater than the threshold, it is directed towards the calculator 14 to effect a new estimation of the position. The comparators 12 and 16 thus constitute with the calculator 14 means 18 for modification of the estimated orientation of the solid S.

The new estimation can be random. It can also be refined according to a correction calculation by the error gradient descent method, which is known. The second comparator can optionally be eliminated. In this case, the estimated value is continually refined until the input of a new measurement value.

The device in FIG. 1 comprises means, for example a memory, for registering the successive estimated values, validated, as a function of successive measurements of the angular position of the solid. The memory M can be part of the calculator and can be localized on the solid S. The successive values enable calculation of the motion of rotation of the solid as well as its speeds and angular accelerations. To start the measuring of a new orientation of the solid, the first test datum generated is advantageously the validated estimated value of the preceding position.

The capture of measurement values by the sensors, and the registering of the estimated values in the memory M, can be rated by a timer H. Contrary to the process of the known type of direct inversion, the capture process of the orientation of a solid according to the invention allows utilization of any number of sensors, provided that this number is greater than the number of variables of angle I to be estimated (the number of variables of angle I to be estimated is between 1 and 3). According to the desired quality of the estimation, a device according to the invention can thus comprise the minimal number of sensors necessary or a number of sensors greater than the minimal number (redundancy).

In an embodiment, the contribution of each sensor can be weighted. A criterion of confidence or weight Cm is then established which is associated with each constituent of the measurement Θ_(m) in order to take the latter more or less into account in the algorithm of angles research. The calculation of a weight Cm is established according to the following rules:

a) the weight Cm has a value equal to 1 by default,

b) the weight Cm takes the value 0 in the event where the provided measurement is an aberrant value (saturation, value translating bad functioning, etc.),

c) the weight Cm has a value equal to 0 when the level of noise measured by the sensor exceeds a certain threshold, an intermediate value varying linearly from 0 to 1 able to be applied for noise values varying from the threshold value to a noise value considered as negligible,

d) the confidence is reduced on the accelerometers if the total acceleration measured moves away in standard from the value of apparent gravity,

e) the confidence is reduced on the magnetometers if the magnetometers register an excessive variation in their standard (the presence of a ferromagnetic object(s) in the vicinity of the sensor can then be suspected).

In the absence of weighting, for iteration done by the calculator 14, the modification of a test angle I is associated with the magnitude $\sum\limits_{n - 1}^{N}\quad\left( {\alpha_{In}\Delta\quad\Theta_{n}} \right)$

where n is the index of a sensor, N is the number of sensors, α_(ln) is a parameter relative to the index sensor n, calculated usually by the gradient descent, ΔΘ_(n) is the spread between the real orientation and the estimated orientation of the index sensor n.

The introduction of a weight Cm, relative to the index sensor n then modifies the expression of the magnitude S_(l) as follows: $S_{I} = {\sum\limits_{n - 1}^{N}\quad{{Cm}_{n}\left( {\alpha_{In}\Delta\quad\Theta_{n}} \right)}}$

In general, the values of a weight Cm_(n) can evolve continuously between the value 1 (total confidence in the measurement made by the index sensor n) and the value 0 (total absence of confidence in the measurement made by the index sensor n, the measurement made by the index sensor n is not taken into consideration).

The subject matter described herein can be directed to devices for sensing motion, sometimes designated by mocap (motion capture) are applied in fields as varied as health, multimedia, mining or geophysical research. In the field of application of video games or simulations, the motions of a user can in effect be registered for controlling virtual reality immersion systems. By way of example, the motions of a player can be registered to control the evolution of a virtual person in a synthesis scene. In the field of health, the motion capture devices can be utilized for positioning a surgical instrument or else for monitoring the evolution of the autonomy of fragile persons by taking their physical activity into account. In the field of portable electronics, the motion capture devices allow apparatus to adapt to the context of utilization. They allow, for example, optimizing of reception, of a portable telephone by recognizing its orientation, or improving the interfaces of personal assistants.

In the description reference is made to the orientation of a solid. The solid does not however form part of the capture device. The orientation corresponds more precisely to that of the sensor or sensors capable of being fixed to the solid. Furthermore, the terms orientation and angular position are utilized as synonyms. Owing to the device it is possible to successively refine estimation of the orientation of the solid.

After one or more modifications of the estimated orientation, the latter converges towards effective orientation of the solid, or, more precisely, towards measured orientation. Therefore, the described device does not require calculation means to establish the orientation or the inclination of the solid on the base of a function (inverse) of the measuring data of the sensors.

The device allows the imperfections of the sensors to be taken directly into account and allows non-linear behaviors of the latter to be set free. By way of example, utilization of sensors with three sensitive non-orthogonal axes is possible.

The correlation between the estimated orientation and the test datum generated can be given, for example, by a direct function f, stated above. This is, for example, a simple function of modeling of the behavior of the sensors. The difference between each of the successive test data and the measuring datum can also be put to profit to control the necessity or not of further refining the estimated orientation. Accordingly, the device can comprise a second comparator with threshold for comparing the difference established by the first comparator with a threshold value and for validating the estimated orientation, when the difference established by the first comparator for a given test value is less than the threshold value. When the difference remains too significant a new estimation of the orientation is undertaken.

The modification means of the estimated orientation and/or the generator means of a test datum can comprise a calculator for establishing a new estimated orientation and/or a new test datum according to a method known as descent of error gradient. Furthermore, the generator means of test data can comprise a calculator for calculating test data as a function of estimated orientation, and as a function of parameters characteristic of a response of the angular position sensor.

The device can comprise one or more angular position sensors sensitive to gravity and one or more angular position sensors sensitive to a magnetic field. In a more general sense other sensors are capable of giving information on their angular position relative to a reference direction of space. For example, there are sensors for measuring a temperature gradient, a pressure gradient, image sensors (visible or thermal). By way of example, the sensor sensitive to gravity can comprise at least one accelerometer and the sensor sensitive to a magnetic field can comprise at least one magnetometer.

In order to measure the angular position in the most complete manner and the best determined, the device is preferably equipped with two sensors each having three axes of sensitivity. An embodiment also concerns a capture device of the rotation motion of a solid comprising an orientation capture device such as described hereinabove and means for registering successive estimations of the orientation of the solid. This is, for example, a memory. The device can also comprise a timer for rating the registration of the successive estimations of the orientation of the solid. The timer also allows speeds and angular accelerations to be established, if necessary. Calculating the motion can take place in the calculator and according to classic laws of kinetics of a solid. 

1. A capture device of the orientation of a solid comprising: a sensor of angular position, capable of being affixed to the solid and of supplying at least a measuring datum related to a real orientation of the solid, wherein the sensor of angular position being sensitive to at least one of gravity or a magnetic field; calculation means for generating test data independent of any measuring datum and related to an estimated orientation of the solid; and means for modifying the estimated orientation of the solid by confrontation of the measuring datum and the test data.
 2. Device as claimed in claim 1, wherein said means for modifying the estimated orientation of the solid comprise a first comparator connected to said sensor and to said calculation means, the first comparator configured to receive the measuring datum and at least a test datum, wherein the first comparator is configured to establish at least a difference between the test datum and the measuring datum.
 3. Device as claimed in claim 2, further comprising a second comparator with a threshold for comparing the difference established by the first comparator to a threshold value and to validate the estimated orientation of the solid, when the difference established by the first comparator is less than the threshold value.
 4. Device as claimed in claim 1, wherein the sensor is sensitive to gravity, the sensor comprises at least an accelerometer.
 5. Device as claimed in claim 1, wherein the sensor sensitive to a magnetic field and comprises at least a magnetometer.
 6. Device as claimed in claim 1, wherein the sensor further comprises two sensors each having three axes of sensitivity.
 7. Device as claimed in claim 1, wherein said calculation means comprises a calculator configured to calculate test data as a function of parameters characteristic of a response of the angular position sensor.
 8. Device as claimed in claim 7, wherein the calculator is localized on the solid.
 9. Device as claimed in claim 1, wherein said means for modifying the estimated orientation of the solid further comprises a calculator for establishing a new estimated orientation of the solid according to an error gradient descent method.
 10. Device as claimed in claim 1, wherein said calculation means comprises a calculator for establishing a new test datum according to an error gradient descent method.
 11. Device as claimed in claim 10, wherein the calculator is localized on the solid.
 12. Device as claimed in claim 1, further comprising means for registering successive estimated orientations of the solid.
 13. Device as claimed in claim 12, wherein said means for registering is localized on the solid.
 14. Device as claimed in claim 13, comprising a timer configured to rate registration of the successive estimated orientations of the solid.
 15. A process for estimation of the orientation of a solid comprising: capturing at least a measuring datum related to a real orientation of the solid, the measuring datum originating from at least one angular position sensor sensitive to at least one of gravity or a magnetic field; establishing test data independent of any measuring datum and related to an estimated orientation of the solid; confronting the test data and the measuring datum; and establishing a new estimated orientation of the solid based on the result of the confrontation.
 16. Process as claimed in claim 15, wherein said capture and said confrontation are repeated until said confrontation reveals a difference between test data and the measuring datum less than a determined threshold.
 17. Process as claimed in claim 15, wherein a correction calculation is made according to an error gradient descent method during the establishment of the new estimated orientation of the solid. 